The present invention relates to a data receiving system to be used for a portable digital telephone set, a car digital telephone set or the like.
Portable telephone systems have been progressively digitalized in the recent years, and development of digital data receiving units has been promoted. Under this circumstance, the use of equalizers in the portable telephone systems is now essential and unavoidable in Europe. Since portable telephone sets are driven by batteries, it is necessary to develop receiving units requiring small power consumption. Therefore, development of data receiving units including equalizers with less operation for signal processing is more important.
Conventional data receiving units will be explained below.
FIG. 1 shows a structure of the main part of the conventional data receiving unit. In FIG. 1, 21 designates a receiving antenna, 22 a receiving filter and 23 an equalizer.
Operation of the above prior-art example will be explained. Referring to FIG. 1, a signal received by the receiving signal 21 is processed by the receiving filter 22 so that a signal of only a desired channel is taken out. This extracted signal becomes an equalizer input 25 and is inputted to the equalizer 23. The equalizer 23 removes a distortion of a transmission path from the signal outputted from the receiving filter 22 and outputs a received data 24 with small error. When the signal is being transmitted after having been modulated by the MSK (Minimum Shift Keying) system or the GMSK (Gaussian-filtered Minimum Shift Keying) system or the like, T, which is the time necessary for transmitting a one-bit signal, becomes T=(1/transmission rate).
Configuration examples of the equalizer used for the above data receiving unit will be explained below with reference to FIGS. 2A to 2C. FIG. 2A shows an example of a fractional interval equalizer. FIG. 2B shows an example of a linear decision feedback equalizer, and FIG. 2C shows an example of a fractional interval decision feedback equalizer. In each of these drawings, 26 designates a delay unit (T/2), 27 a delay unit (T), 28 an amplifier, 29 an adder and 30 a discriminator. A number of taps and an interval of the fractional interval equalizer are different depending on the conditions under which these units are used.
The fractional interval equalizer will be explained. In FIG. 2A, the equalizer input 25 is first applied to the delay units 26 and is stored in a delay line of a fractional interval. The equalizer input is then weighted to compensate for a distortion of a transmission path by the amplifier 28, and is added together by the adder 29. A plus or minus value of an output from the adder 29 is discriminated by the discriminator 30 so that the received data 24 with small error is produced. Since this equalizer has fractional intervals for taps, or tap intervals are sampled in fine fractions, it is possible to handle data of a wide range. Therefore, this equalizer can compensate not only for a distortion due to a multipulse frequency selective fading but also for a fading due to an interference of adjacent waves.
The linear decision feedback equalizer will be explained below. In FIG. 2B, the operation of the equalizer is the same as the operation of the equalizer in FIG. 2A, except that the forward side of the equalizer, or the portion above the adder 29 in FIG. 2B, has a symbolic interval (T) for the delay quantity of the delay unit 27. At the backward side of the equalizer, or at the portion below the adder 29, the received data 24 is stored in the delay unit 27, weighted by the amplifier 28 and is added by the adder 29. Since this equalizer has the backward side, it can reduce error rates of the received data 24 further than an equalizer having only the fractional interval equalizer or the forward side, for a frequency selective fading (when the delayed wave is smaller than the main wave). However, this equalizer can not compensate a distortion due to an interference of adjacent waves.
Next, the fractional interval decision feedback equalizer will be explained. In FIG. 2C, the operation of the forward side is the same as the operation of the fractional interval equalizer shown in FIG. 2A, and the operation of the backward side is the same as the operation of the linear decision feedback equalizer shown in FIG. 2B. Since this equalizer has the backward side, this has the same performance as that of the equalizer in FIG. 2B for a frequency selective fading. Further, since the forward side of this equalizer has fractional intervals, this equalizer can compensate a distortion due to an interference of adjacent waves.
However, according to the above-described prior-art data receiving units, there are the following problems. The fractional interval equalizer is inferior to the decision feedback equalizer in the performance for a frequency selective fading, the linear decision feedback equalizer can not compensate a distortion due to an interference of adjacent waves, and the fractional interval decision feedback equalizer has a large volume of operation because of a large total number of taps involved.